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Abstract
Calcium (Ca2+) has an important structural role in guaranteeing the integrity of the outer lipopolysaccharide layer and cell walls of bacterial cells. Extracellular DNA (eDNA) being part of the slimy matrix produced by bacteria promotes biofilm formation through enhanced structural integrity of the matrix. Here, the concurrent role of Ca2+ and eDNA in mediating bacterial aggregation and biofilm formation was studied for the first time using a variety of bacterial strains and the thermodynamics of DNA to Ca2+ binding. It was found that the eDNA concentrations under both planktonic and biofilm growth conditions were different among bacterial strains. Whilst Ca2+ had no influence on eDNA release, presence of eDNA by itself favours bacterial aggregation via attractive acid-base interactions in addition, its binding with Ca2+ at biologically relevant concentrations was shown further increase in bacterial aggregation via cationic bridging. Negative Gibbs free energy (ΔG) values in iTC data confirmed that the interaction between DNA and Ca2+ is thermodynamically favourable and that the binding process is spontaneous and exothermic owing to its highly negative enthalpy. Removal of eDNA through DNase I treatment revealed that Ca2+ alone did not enhance cell aggregation and biofilm formation. This discovery signifies the importance of eDNA and concludes that existence of eDNA on bacterial cell surfaces is a key facilitator in binding of Ca2+ to eDNA thereby mediating bacterial aggregation and biofilm formation.
Highlights
Biofilms play a crucial role in both medical and non-medical contexts such as harmful bacterial infections and biocorrosion [1,2] and beneficial impacts in applications in bioremediation, bioelectricity production and wastewater treatment [3,4,5]
Recent discoveries reported that extracellular DNA (eDNA) bind with various biopolymers and metabolites produced by bacteria [21,36,37] and provides structural integrity to bacterial self produced matrix and biofilm promotion and stability
Divalent cations like Ca2+ are primarily known for stabilizing bacterial cell walls and promoting ionic cross bridging among bacterial cells
Summary
Biofilms play a crucial role in both medical and non-medical contexts such as harmful bacterial infections and biocorrosion [1,2] and beneficial impacts in applications in bioremediation, bioelectricity production and wastewater treatment [3,4,5]. Recent studies showed that eDNA chelation of divalent cations induces genes involved in modification of bacterial cell surface properties that favour resistance of biofilms to antimicrobial agents and detergents [20]. Divalent cations such as Ca2+ stabilize bacterial cell walls [22] and promote ionic bridging between bacterial cells via interaction between negatively charged cell membranes and other biopolymers in matrix [22,23]. It acts as a sensory ion for gene expression of biofilmassociated growth [26]
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